Quantitative and real time analysis of the elementary composition of materials is of great interest in many fields including geological survey, industrial production, environment diagnostics and on-line control of product quality, for example.
The LIBS technique is often used for the quick analysis of a sample's elementary constituents since it can be applied in situ and give results in real time.
While the LIBS technique is believed well known in the art, it will be summarized hereinbelow.
LIBS is a spark spectrochemical technique that uses a short-pulsed laser (nanoseconds) or an ultrashort pulse laser (picoseconds and femtoseconds) that is focused on a sample to create a microplasma near the surface thereof. The microplasma is a transient event having a peak temperature reaching 10,000 to 20,000 K.
In this environment, a portion of the sample is converted into plasma and the chemical bonds are broken to produce electronically excited atoms and ions. These excited species give off resonant and sharp radiation at specific wavelengths that depend on the constituent element.
By analysing the light emitted by the microplasma within a narrow range (generally from about 200 to about 980 nm) it is possible to identify the constituent elements by their specific emission wavelengths and to measure the concentration of the identified constituent elements by measuring the intensity of the light at their specific emission wavelengths.
LIBS may be considered a real-time procedure since its response time is generally less than a second.
The LIBS event generates a tremendous amount of data and, interestingly, virtually every laser shot produces a usable spectrum. Furthermore, LIBS is very good at analysing small particles.
LIBS can operate at atmospheric pressure while producing useful plasma emission intensities. The actual plasma emission is generally characterized both by a continuum spectrum (generally referred to as bremsstrahlung emission) and by discrete emission lines.
The continuum emission and the discrete emission, from both atoms and ions, decay at different rates. While the continuum emission decays usually within a few microseconds, the discrete emission persists strongly for tens of microseconds. The discrete plasma emission can, therefore, be resolved both spectrally and temporally to yield spectra containing the atomic emission lines corresponding to the atoms present in the plasma volume.
It is generally known that many variables can negatively influence the precision of LIBS measurements. Some of these variables such as the laser properties (wavelength, pulse duration, focusing spot size, etc.) and the detection window (delay time and gate width) can be taken into consideration when the measurements are taken. However, the physical properties of the sample are more difficult to take into account. This problem called “matrix effects” is well known in the art and is a factor that limits LIBS accuracy. Accordingly, the precision of LIBS measurement are generally relatively poor for many applications.
There is therefore a need for improvements in methods and systems for LIBS measurements.